For the production of optically readable information carriers like Audio-CD's, CD-ROM's, Video-CD's or DVD's injection-molding tools are used, which, by means of two opposing and relatively movable mirror-blocks form a cavity (cavity of the injection-molding form). On one of the two mirror blocks there is usually provided a thin, disc-shaped stamper in a removable manner, which during injection-molding of the information carrier embosses the information carrier traces in the information carrier. At its outer border the cavity is usually bordered by a form ring (venting ring), which surrounds the mirror block concentrically.
The form ring protrudes above the planar surface of the form of the mirror block it surrounds, and it is movable relative to this mirror block in axial direction. If the cavity is closed by means of an axial motion of the two mirror blocks towards each other, due to the protruding position of the form ring it contacts the opposing mirror block and closes the form, while the mirror block it encloses concentrically is at a predetermined distance from the other mirror block. The hot plastic is injected under high pressure into the thus provided disc-shaped cavity. After the cavity is filled, the mirror block which is enclosed by the form ring is moved a little towards the other mirror block in order to achieve a high quality of the information carrier, while the form ring remains stationary (see e.g. FIGS. 2 and 3 of WO-A1-99/37471).
This relative motion taking place in this phase of the production process between the mirror block and the form ring enclosing it however gives raise to problems for the following reasons: to make sure that the form ring can slide freely over the mirror block during the relative motion, a certain fit is necessary. If this fit is too loose, during injection plastic may enter the slot and lead to a fin on the formed information carrier disc, which is not desired. If however the fit is too tight, it may, in particular if the mirror block and the form ring are at different temperatures, lead to high values of friction which limit the relative motion, and, because both parts are usually made of steel, may even lead to seizing or jamming of the form ring on the mirror block.
In order to have low friction at relatively tight fit between the mirror block and the form ring in the above mentioned WO-A1-99/37471 it has already been proposed to provide the surfaces of the mirror block and of the form ring which are gliding on each other with a specific coating, which reduces the friction. In this context it is however a disadvantage that the coating of the parts is associated with high expenses, and a coating generally influences the heat dissipation from the cavity.
In EP-A1-0 899 075 a linear ball bearing (75 in FIG. 5) is provided between the two parts in order to assure a good relative mobility between the mirror block and the ring which concentrically surrounds the mirror block. The ring in this case however is not a form ring which forms part of the bordering of the cavity, but it is a guiding ring which only provides centering of the two mirror blocks relative to each other.
In U.S. Pat. No. 5,018,962 an injection molding tool comprising two mirror blocks and a form ring for the production of CD's is disclosed, in which between the form ring and one of the mirror blocks a concentric holding ring (15 in FIG. 1) is provided for the stamper which is sitting on this mirror block. The form ring slides over both mirror blocks if the form is opened or closed. There is however no means provided for the reduction of friction during these sliding processes.
In DE-A1-44 10 015 the possibility is described to provide a bearing between the form ring and the mirror block which it encloses (61 in FIG. 1). How this bearing is actually structured is however not disclosed in this document, there is only the statement that it is a cylindrical bearing, which apparently shall serve to simplify the relative mobility between a movable part of the form half 52 (corresponding to a mirror block) and a swimming element 58 (probably corresponding to a form ring). This “bearing” is provided in a region distanced from the cavity, and the swimming element is in direct contact with the movable form half in the bordering region to the cavity.
In EP-A1-1 120 221 the possibility is disclosed to provide a sliding ring between a mirror block and a form ring which is enclosing this mirror block concentrically, wherein this sliding ring is provided concentrically between the mirror block and the form ring enclosing it. This sliding ring is provided to prevent direct friction between the mirror block and the form ring.